With the spread of mobile telephones in recent years, there have been increasing demands for voice communication and data communication by the mobile telephones not only from outdoors but also from indoors. In order to satisfy such an increase in demands, development of ultra small base stations that can be arranged indoors such as user's houses or small offices has been advancing. The area covered by such a base station is called a femtocell since such an area is extremely smaller than a coverage area of a base station arranged outdoors (hereinafter referred to as a macro base station). The cell means coverage of a base station (communication area that satisfies required quality). In the following description, such an ultra small base station is referred to as a femtocell base station. Examined as an operation form of the femtocell base station is a technique in which only a mobile station that is registered in advance connects to a femtocell base station to perform communication. Further, a femtocell base station may be arranged in such an area including upper floors of a building, or underground areas where radio waves are difficult to reach. Thus, the femtocell base station has attracted an attention as means for extending the coverage.
The femtocell base station is examined to be used in a system like a wideband code division multiple access (W-CDMA), an Evolved Universal Terrestrial Radio Access (E-UTRA: also referred to as a Long Term Evolution: LTE), and IEEE 802.16m. The W-CDMA and the E-UTRA are radio communication standards of mobile telephones. IEEE 802.16m is a radio communication standard of a wireless metropolitan area network (wireless MAN).
When the femtocell base station is used in the W-CDMA, data transmission using a dedicated channel in which transmission power in uplink and downlink is controlled and data transmission using a shared channel in the downlink are performed.
When the femtocell base station is used in the E-UTRA, a scheduler arranged in the base station allocates physical resource blocks (PRBs), whereby data transmission using the PRBs that are allocated is performed. The PRB is a basic unit of the radio resource in the downlink of E-UTRA employing an orthogonal frequency division multiple access (OFDMA), and includes a plurality of OFDM subcarriers in a frequency domain, and includes at least one symbol time in a time domain.
Further, OFDMA is also employed in the downlink of IEEE 802.16m. When the femtocell base station is used in IEEE 802.16m, the scheduler arranged in the base station allocates subcarriers, whereby data transmission using the subcarriers that are allocated is performed.
By the way, a base station in an existing mobile communication network transmits control signals called pilot signals to its coverage area (coverage). The mobile station receives the pilot signals to perform synchronization establishment with the base station, channel estimation and the like, and transmits/receives data to/from the base station. Accordingly, this allows the mobile station to receive the pilot signals transmitted from the base station with high quality, thereby capable of providing high communication quality. The same is applied to the femtocell base station as well.
In the base station in the existing mobile communication network, power saving can be achieved by suppressing transmission of the radio signals including the aforementioned pilot signals as much as possible. For example, a patent literature 1 discloses that a base station receives radio signals transmitted from a peripheral base station, and the base station adjusts its own transmission power using the state of traffic of the peripheral base station and the measurement result of reception power of the signals transmitted from the peripheral base station. More specifically, the base station according to the patent literature 1 reduces or stops the transmission power of an own station when the traffic of the peripheral base station is low and the reception power of the radio signal from the peripheral base station satisfies a predetermined quality.
Further, a patent literature 2 discloses a technical method of adjusting transmission power of a base station as is similar to that in the patent literature 1 described above. More specifically, the patent literature 2 discloses that a complementary base station measures reception power of radio signals transmitted from a peripheral base station to adjust its own transmission power according to the measurement result in order to efficiently cover areas (dead areas) which are not sufficiently covered by the peripheral base stations.
Further, a patent literature 3 discloses a method of optimizing a radio parameter (including transmission power) of a new base station when the base station is newly provided. More specifically, according to the patent literature 3, a mobile station measures reception power of radio waves from the new base station and its peripheral base stations in order to optimize the radio parameter of the new base station. Then, the new base station adjusts the radio parameter of an own station based on the measurement result of the mobile station.
A patent literature 4 discloses a technique related to handover of a mobile station to a femtocell. More specifically, the mobile station disclosed in the patent literature 4 receives radio signals from a peripheral base station, measures reception quality, and acquires a group identifier of the peripheral base station. The mobile station then identifies whether the group identifier that is acquired corresponds to a femtocell base station to which the mobile station itself is able to connect, and transmits the measurement result of the reception quality to the base station that is being connected under a condition that the group identifier relates to the femtocell base station to which the mobile station is able to connect. Accordingly, the start of the handover of the mobile station to the femtocell base station to which the mobile station is not able to connect can be suppressed.